1. Field of the Invention
The present invention relates to a four-wheel drive work vehicle selectively drivable in a standard four-wheel drive mode for driving front wheels and rear wheels at a substantially same speed, an accelerated four-wheel drive mode for driving the front wheels at a higher speed than the rear wheels when the front wheels are steered beyond a predetermined angle or in a two-wheel drive mode for driving only the rear wheels while breaking the power transmission to the front wheels.
2. Description of the Prior Art
An agricultural tractor as one example of the above-described four-wheel drive work vehicle is known from e.g. a Japanese patent laid open under Showa: No. 63-34229 (U.S. Pat. Ser. No. 4,723,623). This agricultural tractor is drivable in the three drive modes: the two-wheel drive mode where only the rear wheels are driven, the standard four-wheel drive mode where the front wheels and the rear wheels are constantly driven at substantially the same speed and the accelerated four-wheel drive mode where the front wheels are driven at a higher speed than the rear wheels when the front wheels are steered beyond a predetermined angle.
More specifically, the two-wheel drive mode and the standard four-wheel drive mode are selected other through a forward-reverse pivotal operation on a mode-switch lever; whereas, the standard four-wheel drive mode and the accelerated four-wheel drive mode are selected through an up-down pivotal operation of the mode-switch lever. Accordingly, if a driver desires to change the vehicle drive mode from the two-wheel drive mode to the accelerated four-wheel drive mode, he must operate the mode-switch lever in two directions successively; namely, operating the lever in the forward-reverse direction and then in the up-down direction. That is to say, in this prior art, the 3-mode switching operation requires two steps of successive operations of the switch lever in the two different directions. This inconvenience has significantly impaired the drivability of the work vehicle.
The present invention attends to such inconvenience and aims at improvement of the drivability, i.e. the drive-mode switching operability of the work vehicle.